String hexToAscii(String s) {
int n = s.length();
StringBuilder sb = new StringBuilder(n / 2);
for (int i = 0; i < n; i += 2) {
char a = s.charAt(i);
char b = s.charAt(i + 1);
sb.append((char) ((hexToInt(a) << 4) | hexToInt(b)));
}
return sb.toString();
}
private static int hexToInt(char ch) {
if ('a' <= ch && ch <= 'f') { return ch - 'a' + 10; }
if ('A' <= ch && ch <= 'F') { return ch - 'A' + 10; }
if ('0' <= ch && ch <= '9') { return ch - '0'; }
throw new IllegalArgumentException(String.valueOf(ch));
}